Amplification control circuits



Sept. 5, 1939. T. M. sHRADl-:R

AMPLIFICATION CONTROL CIRCUITS Original Filed April 18, 1952 V a @uw INVENTOR TERRY M. SHRADER BY ATTORNEY n A C/U D/l. ad. n 111111 m w 7 M m ,4W I. n r A. ma s 2 C V 7 5 m4. Z 0 M/ JM w\ w 07N h. uhhh IM. 5 3/ H ,QJA ll m Patented Sept. 5, 1939 UNITED STATES PATENT OFFICE AMTLIFICATION CONTROL CIRCUITS Terry M. Shrader, West Summit, N. J., assignor to Radio Corporation of America, a corporation of Delaware 19 Claims.

My invention relates to electron discharge devices or thermionic tubes and circuits for utilizing them, and more particularly to thermionic tubes having two or more grid electrodes interposed between the cathode and the plate electrode, and to various circuits and combinations in which such a tube may be used to advantage, the present application being a division of my application Serial No. 605,879, led April 18, 1932 U. S. P. 2,037,396, issued April 14, 1936.

Screen grid tubes, which have two grid electrodes interposed between the cathode and the plate electrode, and pentodes, which have a control grid and screen grid and also a third grid v electrode or suppressor grid interposed between the screen grid and plate electrode and usually connected internally to the cathode, are known in the art. As usually made, both the screen grid tube and the pentode have sufficient capacity between plate and ground to impose some limitations on their use for some purposes, such as with short waves having a length of about five (5) meters, and in both of them the grid electrode adjacent the cathode is used as a control element.

One object of my invention is to provide a multiple grid tube having in general the advantages of screen grid tubes and pentodes, but so constructed that its characteristic can be varied at will to a greater extent than in the tubes heretofore used, and capable of being operated so as to have characteristics approaching those of a triode, with a low amplication factor and low plate resistance, whereby control of gain and of iidelity is improved.

Another object of my invention is to provide an electron discharge device or tube having two or more grid electrodes interposed between the cathode and the plate electrode and with less inter-electrode capacity, and less plate to ground capacity than the multiple grid tubes heretofore used.

Still another object of my invention is to provide various circuits and combinations to utilize the properties and advantages of a tube having three grid electrodes between the cathode and the plate electrode and constructed in accordance with my invention. Various other features and advantages of my invention will appear from the description which follows.

In accordance with my invention the tube has an electron emitting cathode, preferably of the indirectly heated unipotential type, an anode or plate electrode which surrounds the cathode, and three grid electrodes interposed between the cathode and anode and arranged concentrically around the cathode to be passed successively by the electron stream from the cathode to the anode. Each of the electrodes has its own separate and independent terminal on the outside of the sealed envelope or bulb which encloses the electrodes, one of these terminals, for example the terminal for the iirst or control grid nearest the cathode, being mounted on the top of the bulb, and the other terminals on the usual base secured to the opposite end of the bulb.

Such a tube, in which the electrodes can be connected in many diierent ways, and various voltages can be applied to the electrodes independently at will, is a new tool in the high frequency signalling art of great flexibility and adaptability, and with which many new results can be obtained. By the application of suitable voltages to the electrodes, and particularly by operating the rst and second grids as control and screen grids, and the third grid as an independent regulator grid, preferably negative with respect to the cathode, important characteristics of the tube can be varied at will, and the ow of the electron stream from the cathode to the plate electrode can be regulated and modied more completely and in a greater variety of ways than in the screen grid tube or in the pentode.

For example, by applying the proper negative voltage to the third or regulator grid the space charge, or cloud of electrons around the cathode, can in effect be brought out to the vicinity of the second or screen grid, in which case the effect of the control grid near the cathode is decreased, and the tube has characteristics approaching those of a triode as the screen grid becomes effectively, at least to some extent, a cathode. Under these conditions the plate electrode can draw electrons through the third grid alone easier than through all three grids, hence the voltage gain falls off and the plate resistance falls to a low value. With this type of control considerable improvement in the control of gain and of delity is possible.

A tube constructed in accordance with my invention has been disclosed in my said parent patent, and can be used in many ways and in manifold circuit combinations to great advantage. For example, it is very useful as a biased detector, and as an automatic volume control tube for supplying rectified voltage to the grid of a radio frequency amplifier to aifect the bias of the grid to an extent dependent on the strength of the signal voltage impressed on the amplilier. As a radio frequency amplier it is particularly useful, one important field of use being that of gain-fidelity control by automatically impressing on the third or regulator grid a voltage which in response to strong signals broadens the resonance curve to the desired extent.

The novel features which I believe to be characteristic of my invention areV setforth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation., will best be understood` Y by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organi-- Zations whereby my invention may be carried into effect.

In the drawing: l Y

Fig. 1 shows a biased detector circuit employing the invention,

Fig. 2 shows an automatic volume control' circuit embodying the invention,

Fig. 3 shows a receiving system employing the invention, Y

Fig. 4 is a fidelity curve of a circuit embodying the invention,

Figs. 5, 6 and 7 are characteristic curves of a tube employing the invention.

Referring now to the accompanying drawing, wherein similar circuit elements in the different figures are designatedby similar'reference numerals, the tube shown in Fig. 1 has a sealed envelope such as a bulb 20 which is highly evacuated in the usual way, and which encloses the electrodes. The cathode 28, 'of high` electron emissivity and preferably of the unipotential indirectly heated type, is provided with a. heater, such as a reverse wound or double helix of tungsten wire. Y

The first grid electrode or control grid comprises twov side rods on which the grid wire 33 is helically wound. Where the tube is to have a sharp cut-off the grid is .a helix of uniform pitch; where the tube is to have a variable amplication factor and a comparatively long cutoff, the grid hasV a helix of considerably coarser Vpitch near the middle than .at the ends.

VThe second grid, comprises two side rods on which the grid wire 36 is wound. as a circular helix of uniform. pitch, and this second' grid is maintained positive to act as a screen grid.

The third or regulator grid comprises two side rods on which the grid wire 38 is wound in a circular helix. It is preferably so proportioned that when it is at cathode potential the tube has the characteristic of a pentode with the usual suppressor grid.

'Ihe anode 39 is preferably a cylinder of sheet metal, such as nickel, surrounding and coaxial with the grids and the cathode, and carried on side rods. The anode 39 is preferably carbonized, o-r otherwise blackened or roughened. Y

The internal shielding system comprises a rigid and strong support, such` as two. comparatively heavy and rigid vshield rodsr extending from the stem inside the anode and between it and the third grid to the top of the bulb. These rods may be outside the anode, in which case, they may carry shielding means in addition to what is here shown. On the upper end of the shield rods atop shield .42',v preferably made somewhat like a flat bottomed deep metal' cup with a rectangular slot or opening with rounded ends in the bottom, is secured to the shield rods. The top shield is deep enough so that in conjunction with an external shield it is eifectively of considerably greater electrical diameter than the tive.

anode, but mechanically it is of approximately the same diameter. The entire shielding system is at ground potential, due to a connection between one of the shield rods and the cathode sleeve, and thereby a very complete and eifective shielding and a marked reduction in inter-electrode leakage at high frequency isobtained.

A tube constructed in accordance with my invention can be used in many ways. For example, it may to advantage be used as a biased detector in which case desirable results are obtainable by adjusting the'voltage on the third grid to make it either negative, Zero or positive. The tube is also useful as an'automatic volume control tube in which case it acts as a detector so connected that the radio frequency voltages are applied to the input circuit, and the output circuit is so arranged that .a direct Voltage which varies at a sub-audio rate appears across the output circuit and'is fedback to the radio frequency amplier stages and alsoif desired to theY first detector s tageto control the sensitivity ofthe receiver. In thisA case as in the case where the tube is used .as a biased detector the ability to adjust the potential of the third grid. at will is of advantage.

The tube with. the variable pitch grid is particularly useful as a radio frequency amplifier, and when so used the third or regulator grid with its independent and separate terminal permits the attainment of automatic fidelity control as wellas automatic volume control, due in part to the fact that as the third or regulator grid is made more and more negative the plate current shifts more and more to the screen grid, lowering the impedance of the tube, so that the second grid, the` third gridand the plate electrodes act somewhat like'a triode. One explanation of this action is that when the voltage on the third grid isY Zero, the space charge is almost entirely between the cathode and the first or control grid, but as the voltage of the third or regulator grid becomes more negative the space charge around the second grid inside the thirdA or regulator grid becomes moreintense and the tube begins functioning somewhat like a triode.

Theplate resistance at the operating plate voltage decreases progressively with increasing negative voltage on the third or regulator grid until the 4thirdgrid reaches some optimum negative voltagefafter which the plate resistance again rises as the third. grid becomes still more nega- Fundamentally this tube is one in which the impedance can be controlled at will by changing the voltage on the third-or regulator grid. For example, in a tube embodying tlieinventionY and designed to operate at 250 volts onthe plate and volts on the second grid or screen, the impedance can be varied from about one million ohms when the third grid is connected to the cathode to about twenty thousand ohms when the third gridhas` anegative bias of about forty volts.

,The changeof impedance of the tube with change of bias of the third grid permits the tube 'to be used for automatic fidelity control, since if the plate resistance of a radio frequency amplifier canbe made to decrease progressively with increasing strength of the signal better fidelity can be obtained for loud signals while retaining good selectivity for the weaker signal. The resonance curve of a simple selective circuit is peaked, and for Values of conductance greater than the original the curve becomesrflatter in the neighborhood of resonance, indicating that the higher side bands will be transmitted through the circuit with a greater degree of intensity and consequently the higher audio frequencies will be less attenuated. For loud signals the tube may thus be made to automatically cause the tuned circuits associated with it to become broadened in their response and consequently cause the fidelity automatically to become better.

These results Aare desirable with the present alignment of transmitter frequency channels, and they may be obtained by Varying the plate conductance of the tube with the signal Voltage Which can be done very nicely with'a tube constructed in accordance with my invention by using the third or regulator grid. As th'e third grid is made more and more negative the plate resistance of the tube automatically goes lower. The plate resistance is in shunt across a simple selective circuit connected to the tube, and as this plate resistance decreases the equivalent series resistance of the circuit increases, so that the put circuit is similar to its natural response. The

third grid, particularly when its potential is negative or Zero with respect to the cathode, is effec'- tive in reducing fluctuation noises, especially those due to secondary emission. In general,

`making the third grid negative with respect to the cathode tends to reduce the trans-conductance of the tube and cause sharp cut-off.

Y Figure l indicates diagrammatically one circuit in which the tube may be used as a biased detector, which in this particular case has an automatic bias connection. In this circuit the signal input circuit is connected with the control grid 33, the cathode sleeve 28 being connected to an automatic biasing resistor 59 in parallel with a by-pass condenser 60. sothat the bias is dependent upon the current flowing in the plate circuit. The secondv grid 36 is maintained sufiiciently positive by a battery 6I to act as a screen grid, and the anode is connected to a battery 62 through the' output circuit. Upon the third o-r regulator grid 38 is impressed a voltage whichl may at will be made negative, Zero or positive with reference to the cathode and the value of which depends upon the characteristic desired.

This voltage for the third grid 38 may conveniently be derived from a potentiometer comprising a slider 63 cooperating with a resistance 64 connected across the battery 65. The range of the potentiometer may to advantage be from about 40 volts negative to 40 or 50 volts positive. Ordinarily the slider 63 is set at the point which gives the desired characteristics and need not be changed unless conditions change. For a xed bias detecto-r the automatic biasing resistor and by-pass condenser are omitted and the grid is biased by a battery or similar device. The rectification occurs in the usual way due to the relation between the plate current and control grid voltage.

Figure 2 indicates diagrammatically a circuit in which the tube is used for automatic volume control. The tube is connected substantially as in Figure 1 except that the automatic biasing resistor and by-pass condenser are omitted and a iixed bias is putV on the control grid 33 by a biasing battery 66. The receiving circuit, shown conventionally, comprises a radio frequency amplifier stage of the usual construction, and for convenience also indicated diagrammatically as a triode with its grid connected to the input circuit, conventional couplings 6l, a detector, an audio amplifier, and a loud speaker 68. A tube embodying my invention is connected in the circuit to function as an automatic volume control tube, the voltage appearing across the output circuit of the tube being direct voltage at subaudio frequency which is fed back to the amplifier stages, and also if desired to the first detector Stage if the receiver is of the superheterodyne type to control the sensitivity of the circuit. In the diagram this direct voltage is fed back to the input of the radio frequency stage through leads 69 and a high reactance l0, a resistance 'H being introduced into the input circuit as shown.

Figure 3 indicates diagrammatically a receiving circuit with the elements indicated conventionally, and in which both automatic volume control and fidelity control are obtained by applying to a radio frequency amplifying tube, constructed in accordance with the invention, control voltages derived from the output circuit of the combined detector and automatic volume control unit, which may be of any of the Well known types used in the art. In the particular arrangement shown a resistance 12 grounded at one end, and connected across the output leads of the detector and automatic volume control unit, has cooperating with it two sliders 13, one of which is connected through a lead 14 to the third or regulator grid 38 of the tube, and the other through the lead l to the first or control grid 33.

By setting the sliders at different points on the resistance different voltages may be impressed on the first and third grids of the tube, and by properly selecting these voltages both automatic volume control and increased fidelity control may be obtained. If desired the lead 'i5 to the first or control grid can be omitted, and the voltage from the combined detector and automatic volume control unit impressed upon the third or regulator grid only.

Figure 4 shows curves indicating the fidelity control obtained with this tube when used in a suitable circuit. In this figure the curve indicated by 'i6 is the resonance curve over a range of l0 kilocycles of a simple selective circuit tuned for maximum selectivity. When a loud signal is received the bias of the third or regulator grid is automatically made more negative by the Voltage derived from the combined detector and automatic volume control unit, the extent of this change in bias depending upony the strength of the signal. The resonance curve then changes from that indicated by 16 to that indicated by l1. This change increases the iidelity, reducing the selectivity to some extent at the same time, but with loud signals the decrease in selectivity is not important. When the signal is so weak that it produces no effect upon the bias of the third grid the selectivity is at a maximum, which is desirable for the reception of weak signals. The net result is that in general the fidelity of reception with a circuit such as shown in Figure 3 is better .than with the usual circuits of this kind.

Figures 5, 6 and 7 show characteristic curves obtained from a tube constructed as above described and operated at about 250 volts on the plate and 100 volts on the second or screen grid.

Figure 5 shows the effect of' changes in the bias of the third or regulator grid on the plate voltage-'plate current characteristic. When the regulator grid is acting like the usual suppressor grid, with zero bias, the characteristic is substantially as indicated by curve 18. When the regulator grid is biased to 10 volts negative with reference to the cathode, the characteristic is substantially as indicated by the curve '19.` As the negative bias of the regulator grid is increased by l0 volt steps, the characteristics chan-ge as indicated by the corresponding curves 88, 81|, 82, 83 and 84. It will be seen that while the upper part of the curve 'i8 has only a slight slope,'being substantially iiat, the other curves increase in slope as the negative bias of the regulator grid increases until when thenegative bias in this particular tube is about 40 volts negative the slope is as indicated by curve 82. As the negative bias further increases the upper part of the curve again begins to flatten, as indicated by curves 83 and 8i.

Figure 6 shows the effect upon the plate resistance of this particular tube of changes in the negative bias of the regulator grid for different biases on the control grid. From these curves it is apparent that in this particular tube the plate resistance is a minimum in the neighborhood of 40 volts negative bias on the regulator grid. The useful part of these curves is from zero bias to the bias at which this minimum in plate resistance occurs.

Figure '7 shows the effect on the trans-conductance of this tube of a change in the voltage on the regulator grid from zero to a negative bias of about 50 volts, with diiierent xed voltages on the first or control grid. A pronounced negative bias upon the regulator grid exerts a marked effect upon the important characteristics of the tube.

While I have indicated and described several systems for carrying my invention into eiect, itA

will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modications may be made without departing from the scope of my inventionas set forth in the appended claims.

What I claim is:

l. An electron discharge circuit comprising a cathode, a cylindrical anode, an output circuit connected to the cathode and anode, and three cylindrical grids interposed between and coaxial with said cathode and anode, means for impressing a positive potential upon the intermediate grid, a signal input circuit between the cathode and the grid nearest the cathode and means for independently impressing upon the grid nearest the anode a potential of suiiicient negative value to appreciably aect the electron stream between the intermediate grid and the anode and substantially change the anode resistance of the network.

2. An electron discharge network comprising a cathode, a cylindrical anode, an output circuit connected to the cathode and anode, and three cylindrical grids interposed between and coaxial with said cathode and anode, means for impressing a positive potential upon the intermediate grid, a signal input circuit between the cathode and the grid nearest the cathode and means for impressing upon the grid nearest the anode a potentialsufiiciently negative with respect to said cathode to vary the anode resistance of the network.

3. In a receiving circuit, the combination with an electron, discharge amplifier comprising a cathode, an anode, an output circuit between the cathode and anode and three successive grids interposed between said cathode and anode, of l means for impressing a positive potential upon the intermediate grid, a signal input circuit between the cathode and the grid nearest the cathode and means responsive to the anode current of said amplifier for automatically impressing 1 upon the grid nearest the anode a negative biasing potential dependent upon the anode current of said amplifier.

4. In a receiving circuit, the combination with an electron discharge amplier comprising a 1 cathode, an anode, andv three successive grids interposed between said cathode and anode, of a signal input circuit between the cathode andv the grid nearest the cathode, meansV for impressing a positive potential upon theintermediategrid, an output circuit between the anode and cathode, and means for automatically varying the bias potentials cf the grids other than the intermediate grid by different amounts in response to the anode current of said amplifier.

5. A signal transmission network comprising an electron discharge tube provided with at least a cathode, an anode, a signal input electrode, a positive screen and an auxiliary electrode between the screen and anode, a signal` input circuit connected to the cathode and inputelectrode, an output circuit connected to the anode, and means for varying the potential of at least the auxiliary electrode over a relatively wide potential range which is negative with respect to the cathode substantially to regulate the eiective anode resistanceV oi"V the tube.

6. A signal amplier transmission network comprisingy an electron discharge tube provided' with at least a cathode, an anode, a signal input 44] electrode, a positive screen and an auxiliary electrode between theY screen and anode, a signal input circuit connected to the cathode and input electrode, an output circuit connected to the anode, andv means for varying the potential of at least the auxiliary electrode over av potential range which is negative with respect to the cathode to regulate the effective anode resistance of the tube in such a manner that the selectivity of the amplifier decreases with increase of nega- 5 tive bias of said auxiliary electrode.-

7.A signal transmission network` comprising an electron dischargeV tube Yprovided with at least a cathode, an anode, a signal input electrode, a

positive screen and an auxiliary electrode between 65 the screen and anode, a signal input circuit connected to the cathode and input electrode, an output circuit connected to the anode, and means for varying the potential of at least the auxiliary electrode and said input electrode over a wide 6 potential range which is negative with respect to the cathode to regulate the eiective anode resistance of the tube. Y

8. In a signal reception network including a tube provided with an electron emission element, an anode and a signal .input electrode, a positive screen grid between the signal electrode and the anode, an auxiliary electrode adjacent the screen grid and the anode at a negative potential, said screen and negative electrode cooperating to pro- 7 vide a virtual cathode between the signal electrode and the anode by virtue of a. high value of said negative potential, a signal input circuit connected to the input electrode, and an. output circuit connected to the. anode, and means for varying the negative potential of the auxiliary electrode to regulate the gain of the tube.

9. In a signal reception network including a tube provided with an electron emission element, an anode and a signal input electrode, a positive screen grid between the signal electrode and the anode, an auxiliary electrode adjacent the screen grid and the anode at a negative potential, said screen and negative electrode cooperating to provide a virtual cathode between the signal electrode and the anode, a signal input circuit connected to the input electrode, and an output circuit connected to the anode, and means, responsive to amplitude variations in received signals, for adjusting the potential of the auxiliary electrode over a negative range to regulate the gain of the tube.

10. In a signal reception network including a tube provided with an electron emission element, an anode and a signal input electrode, a positive screen grid between the signal electrode and the anode, an auxiliary electrode adjacent the screen grid and the anode at a negative potential, said screen and negative electrode cooperating to provide a virtual cathode between the signal electrode and the anode, a signal input circuit connected to the input electrode, and an output circuit connected to the anode, a signal transmission tube, means for transmitting the output of the last tube to said signal input circuit, and a connection between the said output circuit and a gain control electrode of the signal transmission tube.

11. In a radio receiver, a signal frequency amplifier, a demodulator, a tuned network coupling the amplifier and demodulator, said amplifier including a pentode tube provided with a suppressor grid between the screen grid and plate thereof, and means, responsive to received signal amplitude changes, connected to the suppressor grid to vary the selectivity of said network in a sense opposite to said amplitude changes.

12. In a radio receiver, a signal frequency transmission tube of the type including a cathode, a plate, a signal control grid and at least one cold electrode disposed adjacent the plate, a resonant signal selector network connected in the plate circuit of the tube, said network being tuned to the desired signal frequency, and means for varying the potential of the cold electrode negatively and over a range sunicient to substantially change the plate impedance of said tube and thereby vary the selectivity of said selector network in a sense opposite to the signal amplitude changes.

13. In combination with a tuned signal selector circuit, a tube of the pentode type, said circuit being in the plate circuit of the tube, means, responsive to signal amplitude variations, for varying the bias of the pentode suppressor grid over a wide range of bias values, said range being such as to produce changes in plate impedance of the tube suiicient in magnitude to substantially vary the selectivity of said tuned circuit.

14. A method of regulating the ampliiication of signal Waves which consists in impressing the waves upon the signal grid of an amplifier tube having an effective plate current-signal grid voltage characteristic whose slope is dependent upon the potential of a gain control grid, separating the signal and gain control grids by a positive screening field, and varying the bias of said gain control grid in response to the variations in the amplitude of received signal waves.

15. An amplilier including a tube having input and output circuits, said tube having at least a cathode, anode, a positive screen grid, a signal grid between the screen grid and cathode, and a gain control grid between the anode and screen grid, means for varying the negative bias of the gain control grid over a range of voltage values, and a modulated signal carrier wave input circuit connected to the signal grid.

16. An amplier including a tube having input and output circuits, said tube having at least a cathode, anode, a positive screen grid, a signal grid adjacent the screen grid, and a gain control grid between the screen grid and anode, means for varying the negative bias of the gain control grid over a range of values, and a source of signals connected to the signal grid, and said bias varying means including a device adapted to produce from said signals a direct current potential component proportional in value to the signal amplitude.

1'7. In combination with a source of wave energy, an amplier having at least three grids, one of the grids being a positive screen, the other grid being a gain control grid, and the remaining grid being coupled to the said wave source, said gain control grid being disposed between the screen and plate, said tube having an effective plate current-wave grid voltage characteristic whose slope depends on the bias of said gain control grid, the potential range of the wave grid available for waves being substantially independent of the gain control grid bias, and means for varying the bias of the gain control grid over a range of negative values to regulate the ow of current in the plate circuit of said tube.

18. A method of regulating the ampliiication of signal waves which consists in impressing the waves upon the signal grid of an amplier tube having an effective plate current-signal grid voltage characteristic whose slope is dependent upon the potential of a gain control grid, electrostatically separating the signal and gain control grids for alternating potentials by a positive screening field, and varying the bias of said gain control grid.

19. In a radio receiver, a plurality of cascadecoupled amplifier tubes, at least one of said tubes including a suppressor grid, means for developing a control voltage in response to an incoming signal, and means for applying said control voltage to said suppressor grid.

TERRY M. SCHRADER. 

